1. Field of the Invention
The present invention relates to an inspection device and method for inspecting foreign matters on reticles or masks used for printing circuit patterns on semiconductor wafers in an LSI production process, for example, on planar substrates such as semiconductor wafers on which circuit patterns are formed.
2. Description of Related Art
In inspecting foreign matters on the surface of reticles or masks, circuit patterns themselves have been increasingly falsely detected as foreign matter, since the recent circuit patterns being formed on the reticles or masks have become highly integrated and miniaturized along with a high integration of LSIs and the like. In order to prevent such kind of false detection as mentioned above, many improvements of apparatus, for example, a use of polarized light, a change in arrangement of a detector, and arranging a spatial filter in front of the detector, have been conventionally taken.
Japanese Laid-Open Patent Application No. 1-245136 describes a foreign matter inspection apparatus including a laser beam source for irradiating, while scanning, a laser beam having a certain polarization angle onto the surface of an inspection substrate; an analyzer for transmitting polarization components of two different polarization angles of scattered/diffracted light generated on the surface of the inspection substrate along with emission of the laser beam; and a detection device having two detectors (a main detector and a reference detector), each of which is configured to be capable of detecting intensity thereof. The inspection apparatus determines the existence of foreign matter by means of a method called a polarization differential method, in which the intensities of the polarization components of two different polarization angles detected by the two detectors respectively are compared, and the ratio and/or the sign of the difference between them are determined to thereby determine whether foreign matter exists or not.
FIG. 10 is an illustration of signal waveforms for explaining an example of signal processing using a polarization differential method. In FIG. 10, Imp1 indicates an intensity signal of a polarization component of a certain polarization angle to the surface of an inspection substrate (hereinafter referred to as an M polarization component), and Irp1 indicates an intensity signal of a polarization component of a different polarization angle from the aforementioned angle to the surface of the inspection substrate (for example, orthogonal to the M polarization component, hereinafter referred to as an R polarization component). Ama1, Amb1, Ara1, and Arb1 indicate representative signals caused by foreign matter, and Pm1 and Pr1 indicate signals caused by circuit patterns formed on the inspection substrate.
As shown in FIG. 10, a signal Pr1 caused by a circuit pattern is detected at an extremely high level, and a signal Pm1 caused by the same circuit pattern is detected at a low level. On the other hand, as for signals Ama1 and Ara1 caused by foreign matter, the signal Ara1 tends to be detected at a lower level than the signal Ama1, and for signals Amb1 and Arb1 caused by another foreign matter, both signals Amb1 and Arb1 are detected at a similar level.
Accordingly, the gain of the intensity signal Irp1 is adjusted so as to make the signal Pr1 of the R polarization component and the signal Pm1 of the M polarization component approach a similar level, and the difference Dp1 between the signal Irp1′ and Imp1 is calculated. When the difference Dp1 is not more than a predetermined threshold level Th, the signal Imp1 is removed as noise, and if the difference Dp1 exceeds the threshold value Th, the signal Imp1 is outputted as foreign matter signals Ada1 and Adb1. In general, the signal Pr1 caused by a circuit pattern is sufficiently large comparing with the signal Pm1, whereby it is desirable to set a gain of the intensity signal Irp1 to 1/10 to 1/100 times the gain of the intensity signal Imp1.
Japanese Laid-Open Patent Application No. 9-218163 shows an apparatus in which a first signal processing line with lowpass characteristics and a second signal processing line without lowpass characteristics are provided in parallel with each other on the output side of a detector, and the difference between outputs from these two signal processing lines is calculated, whereby scattered/diffracted light from a circuit pattern is discriminated from scattered/diffracted light from foreign matter (hereinafter, this discriminating method is called a method using a lowpass difference).
That is, in the method using a lowpass difference, signals caused by a circuit pattern are formed such that a high frequency signal is superposed on a trapezoidal or rectangle (low frequency) signal. Therefore, this signal is branched into two systems, and one is inputted into a circuit with lowpass characteristics and the other is inputted into a circuit with delay characteristics so as to delay the signal through the circuit with lowpass characteristics to thereby synchronize both signals. Then, the difference between the signals is calculated, whereby it is possible to detect only a signal with a high frequency caused by foreign matter.
However, in the case of using the polarization differential method as described in Japanese Laid-Open Patent Application No. 1-245136, since polarization characteristics were used, it is not a sufficiently effective means in the case of, for example, a circuit pattern configuration in which scattered/diffracted light from the circuit pattern causes multiple diffraction/scattering effects.
FIG. 11 is an illustration of signal waveforms for explaining another example of performing signal processing, using the polarization differential method, on signals from a surface of an inspection object which may cause multiple diffractions and multiple scattering. Imp2 indicates an intensity signal of the M polarization component, and Irp2 indicates an intensity signal of the R polarization component. Further, Ama2, Amb2, Ara2 and Arb2 are signals caused by scattered/diffracted light due to foreign matter existing on the inspection object, and Pm2 and Pr2 are signals caused by a circuit pattern formed on the inspection object.
As shown in FIG. 11, when scattered/diffracted light from a circuit pattern causes multiple diffraction/scattering, the signal Pr2 caused by the circuit pattern is detected at a higher level than the signal Pm2. However, the difference between them is not large enough as that of the example shown in FIG. 10. Therefore, even when the signals Ama2, Ara2, Amb2 and Arb2, caused by foreign matter, are detected at the similar level to those of FIG. 10, the signals Ara2 and Arb2 caused by foreign matter become larger comparing with the signal Pr2 caused by the circuit pattern.
Thus, when the level of the signal Pr2 caused by the circuit pattern is adapted to the level of the signal Pm2 as shown with the reference numeral Irp2′, the signals Ara2′ and Arb2′ caused by foreign matter reside somewhat. Therefore, in the difference Dp2 between both signals Irp2 and Imp2, if the signals Ada2 and Adb2 caused by foreign matter become small so as to be not more than the threshold value Th, they are incorrectly determined as noise signals and removed. In other words, there has been a possibility that the signal Adb2, caused by foreign matter scattering relatively strong light even in the polarization differential method, becomes so small that foreign matter is missed.
Further, there are signal forms according to pattern alignment shown in FIGS. 12(A) and 12(B). In the case of performing signal processing using the lowpass difference as shown in Japanese Laid-Open Patent Application No. 9-218163, the effect is low for a circuit pattern structure of a non-dense arrangement shown in FIG. 12(B), whereby it has not been sufficiently effective for all of the various circuit patterns that may be tested.
FIG. 12(A) shows the signal from dense patterns. When plotting the time along the horizontal axis and the signal intensity along the vertical axis, the signal from the fine patterns comprises a rectangular form. On the other hand, the signal from non-dense aligned patterns or particles exists independently, showing the sharp signal plot as shown in FIG. 12(B). Im3 indicates an exemplary signal detected when the inspection object substrate is inspected. As shown in FIG. 12(B), the inspection object substrate 50 includes a circuit pattern 51 easily shining with a laser beam, and a circuit pattern 52 that is not easily shining.
When the inspection object substrate 50, in which the easily shining circuit patterns 51 are distributed in a non-dense manner, is inspected, locally acute signals 53 caused by the circuit patterns 51 are detected as shown in FIG. 12(B). Even though signal processing using a lowpass difference is applied to such signals 53, it is still difficult to remove these signals 53, whereby there is a possibility to detect them as erroneously caused by foreign matter.
Further, even in a case of a relatively large circuit pattern being formed, a leading edge of a signal caused by the circuit pattern includes a component with high frequency, whereby there may be a case where it is not removed completely by signal processing using the lowpass difference. Therefore, there has been a possibility that the leading edge of a signal caused by the circuit pattern is detected as foreign matter in error. Consequently, it is required to increase the upper limit of frequency to be cut by the signal processing using a lowpass difference, which may cause the inspection to miss foreign matter.
The present invention has been developed in view of the aforementioned matters.